1. Field of the Invention
The present invention relates to electronic imaging systems and, more specifically, to correcting projected or displayed images.
2. Background Information
There are a wide-variety of digital image projectors that are currently available. Most digital projectors include a video decoder and a light engine. The video decoder converts video data received by the projector, e.g., from the display connection of a personal computer (PC), into pixel and color data. The pixel and color data is then supplied to the light engine, which converts that data into the actual projected image. The light engine includes a lamp, optics and logic for manipulating the light in order to generate the pixels and color.
There are three different types of technologies utilized by the light engines of today's projectors: Liquid Crystal Display (LCD), Digital Light Processing (DLP) and Liquid Crystal on Silicon (LCOS). An LCD light engine breaks down the light from a lamp into red, green and blue components. Each color is then polarized and sent to one or more liquid crystal panels that turn the pixels on and off, depending on the image being produced. An optic system then recombines the three color signals and projects the final image to a screen or other surface.
DLP technology was developed by Texas Instruments, Inc. of Dallas, Tex. A DLP light engine directs white light from a lamp onto a color wheel producing red, green, blue and white light. The colored light is then passed to a Digital Micromirror Device (DMD), which is an array of miniature mirrors capable of tilting back-and-forth on a hinge. Each mirror corresponds to a pixel of the projected image. To turn a pixel on, the respective mirror reflects the light into the engine's optics. To turn a pixel off, the mirror reflects the light away from the optics.
A LCOS light engine combines LCD panels with a low cost silicon backplane to obtain resolutions that are typically higher than LCD or DLP projectors. The LCOS light engine has a lamp whose light is sent to a prism, polarized, and then sent to a LCOS chip. The LCOS chip reflects the light into the engine's optics where the color signals are recombined to form the projected image.
Oftentimes, a projector is positioned relative to the screen or other surface onto which the image is to be displayed such that the projector's optical axis is not perpendicular in all directions to the screen. Sometimes, for example, even though the projector is set up directly in front of the screen, the optical axis is nonetheless angled up (producing an image above the projector) or down (producing an image below the projector), such as from a ceiling mounted projector. The resulting image that is projected onto the screen has a trapezoidal shape, and the distortion is known as the keystone-effect. In other arrangements, the optical axis of the projector is not only angled up or down, but is also angled to the left or right. Here, the resulting image is a polygon, and the distortion is known as the oblique-effect. In addition to being non-rectangular in shape, the projected images also suffer from variations in the luminance or brightness level. Specifically, those portions of the projected image that are closer to the projector appear brighter, while those portions that are further away appear dimmer. Such non-uniformities in luminance further reduce the quality of the projected image.
Some projectors include mechanisms for correcting keystone distortion in the vertical direction only. These mechanisms typically achieve this correction by one of two ways so that all lines appear to have the same length: (1) increase the subsampling of higher lines, or (2) scaling scan lines. These mechanisms do not, however, correct for the non-uniformity in luminance that also occurs when the projector is positioned such that the screen is not perpendicular to the projector's optical axis. The luminance non-uniformity of an obliquely projected image can become more pronounced when a “composite” image is created by multiple projectors whose individual obliquely projected images are tiled together, e.g., in a 4 by 5 pattern, to form the composite image.
Accordingly, a need exists for correcting luminance non-uniformity resulting from the optical axis of a projector being non-perpendicular to the screen.